1. Technical Field of the Invention
The present invention relates to power transfer devices for transferring power between two component parts and, more particularly, to a rotational power transfer device, having first and second rotary members operative to interrupt a transfer of a rotational power when either the first and second rotary members is exerted with the rotational power exceeding a given value, and related manufacturing method.
2. Description of the Related Art
In the related art, an attempt has heretofore been made to provide rotational power transfer devices that are operative to interrupt a transfer of a rotational power in excess. One example of these devices is disclosed in, for instance, Japanese Patent Application Publication No. 5-171168. In this related art, the rotational power transfer device is employed in a starter and includes a clutch gear and a barrel. A lubricating layer, composed of a manganese phosphate film and a lubricating film, is formed on an inner periphery of the clutch gear and an outer periphery of the barrel. The lubricating film is composed of a mixture of molybdenum disulfide and resin. The clutch gear is press fitted to the barrel with the inner periphery held in face-to-face relation to the outer periphery of the barrel. In addition, the clutch gear and the barrel are so set such that as a rotational power is exerted between the clutch gear and the barrel at a value beyond a given value, a slippage occurs between these component parts.
With the starter employing such a power transfer device, a rotational power of an armature is transferred to the clutch gear via a drive gear and an idling gear. The drive gear, the idling gear and the clutch gear are coated with lubricating oil such as ester family oil or α-olefin family oil with a low frictional coefficient and high permeability with a view to preventing the occurrence of a wear. Lubricating oil scatters to surrounding areas with the rotation of the gears. It has been turned out that as scattered oil adheres onto the barrel and its vicinity and permeates to between the clutch gear and the barrel, a cleavage fracture takes place in molybdenum disulfide that form the lubricating film with a resultant increase in the frictional coefficient. As the frictional coefficient increases, there occurs an increase in the rotational power at which the slippage is caused to occur between the clutch gear and the barrel. Under such a situation, the clutch gear and the barrel become hard to absorb the rotational power in excess resulting from an impact occurring when the pinion is brought into meshing engagement with a ring gear of an internal combustion engine. This results in a fear of causing the gears and the armature to rupture.